In the past, in an inkjet head provided with a nozzle for ink at a distal end of an ink channel and configured to discharge ink droplets from the nozzle and form an image, the ink dries in a slim nozzle if the ink is not discharged for a while. In other words, in some cases, clogging of the nozzle and a discharge failure of the ink occur because thickened ink is held up in the nozzle. Therefore, there are various drying preventing methods. As a representative method, there is a method of covering the nozzle with a nozzle cap to block the nozzle from the outside air when an ink jet reaches a nonprinting area. There is also a method of sucking the ink in the nozzle with sucking means provided on the outside after covering the nozzle with a nozzle cap in which an ink absorbing pad is provided.
Further, as a method of preventing the clogging of the ink and the discharge failure of the ink, there is disclosed a technique for performing, during a period of movement of a carriage performed prior to a printing operation, an idle driving operation for causing microvibration of ink meniscuses of nozzles with pressure generating means.
However, in the method of simply covering the nozzle of the inkjet head with the nozzle cap, unless air tightness of the nozzle cap is considerably kept, the drying of the nozzle cannot be prevented. Therefore, the method is insufficient as a nozzle drying preventing method. In the method of sucking the ink in the nozzle to the outside with the sucking means, it is necessary to powerfully suck the ink hardened by drying in the nozzle. Therefore, consumption of the ink increases.
As explained above, in the inkjet head, the clogging of the nozzle occurs because of an increase in ink viscosity in the nozzle if an idle period of printing is long. Even if the increase in the ink viscosity does not result in the clogging, because of the length of the printing idle time, discharge of ink droplets is not normally performed and image quality is deteriorated.
As measures against this problem, there is known a method of discharging thickened ink in the nozzle by performing preliminary discharge unrelated to printing. However, in this method, excess consumption of the ink due to the preliminary discharge increases.
In the method of reducing ink thickening in the nozzle by performing the idle driving for causing microvibration of meniscuses, since the thickened ink is not discharged to the outside, clogging of the nozzle cannot be sufficiently removed depending on transition of a state in which the inkjet head is used. This idle driving processing is performed only in an accelerating and decelerating region, which is a period in which the inkjet head is moved from a home position to a printing position, and idle driving processing corresponding to a printing state is not performed. Therefore, an interval of the preliminary discharge cannot be set longer than the period.